This invention relates generally to solar modules and more particularly to a thin film solar module in which a thin film photovoltaic structure is fabricated on low strength untempered glass and laminated to a high strength glass support to provide a module structure having strength appropriate for use in unprotected terrestrial applications.
References which are relevant to the present invention include U.S. Pat. Nos. 4,461,922 issued to Gay et al on July 24, 1984 and 4,517,403 issued to Morel et al on May 14, 1985. Both of these patents are hereby incorporated by reference for all purposes. These patents each describe a known amorphous silicon thin film photovoltaic structure which forms a part of the preferred embodiment of the present invention. In addition, each of these patents illustrates a basic concept of prior art solar module structures. In such modules, a high strength glass substrate has been used as the primary structural element on which a photovoltaic structure is fabricated, in case of thin film, or to which photovoltaic cells are laminated, in the case of single crystal devices. The glass also forms the light receiving face of the finished module, that is the surface which is exposed to ambient sunlight. Such exposure also means that this face of the device will be exposed to other normal terrestrial weather conditions such as high winds and hail storms. Thus, in addition to supporting the static loads, that is the weight of the various components, the glass substrate must be quite impact resistant. The required strength has been found available in conventional tempered window glass which is, of course, designed for exposure to most of the same weather conditions.
In developing processes for the commercial manufacture of thin film photovoltaic devices, several problems have been encountered. Thus, the conventional wisdom was that the module structure would be very simple since the thin film structure could simply be deposited upon the tempered glass which forms the primary physical structure of the module. However, many of the thin film processes involve temperatures which are sufficiently high to either reduce the tempering in the glass or to actually cause it to break during processing. For example, one method of tempering glass involves heat treatment. It is relatively obvious that reheating of the glass to a temperature near the original heat treating process can relieve the intentionally induced internal stresses which achieve the desired tempering. In another words, the additional heating can to some extent anneal the glass substrate thereby destroying the tempering.
Another problem has been encountered which may be of even more commerical importance. One of the primary advantages of thin film solar devices is the ability to fabricate them on an essentially continuous basis or at least on very large glass substrates. In the past, one of the primary cost limitations on single crystal solar devices was the fact that individual cell size was limited by the maximum diameter to which a silicon crystal could be grown. This was typically in range of four to five inches in diameter. Large modules were therefore necessarily fabricated by assembling a large number of individual cells and laminating them to a support structure. This, of course, required considerable manual labor and was considered an unavoidable cost. The newer thin film devices can theoretically be fabricated on any size glass substrate. The original production efforts have used glass substrates having the dimensions of the desired finished product, for example one foot by one foot or one foot by four foot. However, such arrangements do not take fully advantage of the ability to deposit thin film structures on even larger glass substrates. For example, additional cost reductions could be achieved in manufacture of one foot by one foot modules if the devices were deposited on glass substrates having dimensions of four feet by four feet. In addition, numerous low power applications of thin film photovoltaic structures have been identified, for example power sources for pocket calculators. These applications require very small finished devices which do not need to be tempered since they are used in protected environments at all times. However, it is still desirable to manufacture such small devices on very large substrates to reduce costs.
Thus, it is seen that for all types of thin film photovoltaic devices, production costs can be reduced by fabrication of the devices on glass substrates larger than the desired finished product. After the photovoltaic structures are completely formed on the substrates, they can be easily cut to final dimensions provided that the glass substrate is not tempered. In addition, it is desirable that all glass substrates have essentially the same thickness so that only one set of processing equipment will be required in the factory to again achieve cost reduction based on scale of the operation.
However, for high power terrestrial applications, the finished modules need to have impact resistant front faces as discussed above. In addition it is highly desirable that the glass sheet on which a thin film device is fabricated also be the sheet which forms the light receiving face of the module to reduce losses caused by absorption of incoming light. These requirments have appeared to be totally incompatible with the cost reduction goals discussed above.